Tunnel field-effect transistors (TFETs) are typically recognized as successors of metal-oxide semiconductor field-effect transistors (MOSFETs) because of their ability to achieve steep subthreshold slope (and of their resulting low off-currents. Due to their transport mechanism based on band-to-band tunnelling indeed, the TFET subthreshold swing can be less than 60 mV/dec (at room temperature), which is the physical limit of conventional MOSFETs, such that potentially lower supply voltages can be used. Different TFET integration approaches exist for both horizontal and vertical technologies.
However, due to the limitation of the tunnelling probability and the tunnelling area for the source junction, the TFET is faced with a problem of small on-state current, which is far less than that of the conventional MOSFET, and this greatly limits the application of the TFET. This is because TFET current is based on band-to-band tunnelling (BTBT) and not on thermionic current. The TFET current may even saturate or drop while increasing VG, especially at low VD. The TFET on-current saturation is related to source depletion. Source depletion may increase the tunneling distance and limit on-current and create a barrier between the ungated source and part of the core-channel (effectively the source) under the gate in a line tunneling TFET.
For example, in case of N-TFET, a more positively charged VG tends to deplete the p-type source. In case of P-TFET, a more negatively charged VG tends to deplete the n-type source when the device is switched on, such that the electric field at the tunnelling junction is not sufficiently large when the TFET turns on, causing the sub-threshold slope of the TFET to be degraded relative to the theoretical value. Therefore, it has become an important issue of the TFET that how to change the source junction configuration in order to supress source depletion.